Chromosome segregation during meiosis places one chromosome, either the maternal or paternal copy, in each gamete. Accurate segregation of genetic material requires that the parental chromosomes interact with one another and exchange genetic information through the formation of crossovers. Crossover formation requires that chromosomes undergo an orchestrated series of morphological transformations. Failure to make crossovers leads to errors in meiotic chromosome segregation, and consequently to infertility and congenital birth defects. The research proposed here will use nematodes and budding yeast to address the mechanisms of chromosome interaction and segregation by employing experimental approaches that take advantage of the unique features of these model systems, including direct imaging of chromosome dynamics in living animals. The molecular mechanisms that allow chromosomes to form, regulate, and respond to crossovers are poorly understood. This proposal probes three aspects of meiotic chromosome organization and dynamics. The first project will explore how the Synaptonemal Complex (SC)?a conserved structure that assembles between homologous chromosomes and regulates the distribution of crossovers?implements chromosome-wide regulation. This work builds on the recent understanding that the SC, despite its ordered appearance when visualized by electron microscopy, is a liquid-like phase-separated compartment. A novel mutagenesis and genetic screening strategy will isolate separation-of-function mutations that perturb the liquid properties of the SC, and in that way assign specific functions to SC components. In addition, poorly understood chromosome- wide SC dynamics will be directly visualized for the first time using long-term live-imaging. A mechanistic understanding of these dynamics will explain how the meiotic program responds to karyotype abnormalities, and how the SC regulates, and responds to, crossovers. The second project focuses on the reorganization of the chromosomes that transforms crossovers?a local exchange of DNA strands?into the connections that hold the parental chromosomes together during the meiotic divisions, and promotes their correct segregation into gametes. A novel way to label only one of the two parental chromosomes and to visualize its morphology throughout meiosis will rely on the advantageous organization of chromosomes in the C. elegans gonad and on super-resolution microscopy. The third project addresses the organization of meiotic chromosomes as loops of chromatin that are anchored at their base to a proteinaceous axis. This conserved chromosome organization is integral to the meiotic program, but the mechanisms regulating its assembly and dynamics remain enigmatic. A novel technique to obtain a high-resolution description of chromosome conformation and dynamics in budding yeast will take advantage of an emerging technology to sequence very long molecules of DNA. This technique could be widely applied to probe chromosome organization in other cellular processes.